Rendered viewport shading in blender 2.8 is low-res on Retina screens
This is a feature, which actually originated from using high resolution screens.
(Other users actually want to increase the pixel size.)

With highresolution screen viewing a small portion on your display, already requires lots of pixels to be rendered. If you just want to have a rough idea of the color and shapes, you don't need a highres version.


Go to the Properties Panel.
Go to the Scene Tab (the second from top).
Go to the Performance section.
Go to the Viewport subsection.


Change the Pixel Size attribute to 1x. If it is set to automatic, Blender probably realizes you are using a high resolution screen and adjust the preview pixel size for better performance.